BARRIER COMPLEXES IN THE DEEP CREEK MOUNTAINS, UT AS G.K. GILBERT’S “INTERMEDIATE SHORELINES” OF PLEISTOCENE LAKE BONNEVILLE

Located on the border of Utah and Nevada, the Deep Creek Mountains are part of Pleistocene Lake Bonneville’s western boundary and relatively undocumented in terms of Quaternary stratigraphy and geomorphology. The highest mountain range on the western side of the basin offers a unique opportunity to study the interactions of the alluvial systems before, during and after the presence of Lake Bonneville. Preserved along the range is a large barrier complex with spit-bar elements as well as “intermediate shorelines” or embankments as described and puzzled over by G.K. Gilbert. The main focus of this project is mapping the large barrier complex located at the bottom of Reilly Canyon, which incises a well preserved Tertiary-Quaternary (T-Q) alluvial fan. We describe the significance of these intermediate shorezone features by reconstructing the transgressive and regressive states of Lake Bonneville in this portion of the basin by observing a number of geomorphic features and sediment exposures. These include bedded marls along with well-preserved regressive Provo sediments depicting waning post Bonneville Flood stages along with a lagoon sequence located at the base of Reilly Canyon that was present as the barrier complex was aggrading. New radiocarbon AMS analyses from in situ gastropods provide dates to discern the history of the flood in this area and add to the numerous other dates from throughout the basin. Ground penetrating radar, GIS analysis and fluvial modeling inform a general chronology for the evolution of the barrier complex overtime, along with a detailed depositional history for the area. Preliminary volumetric estimates show that the barrier complex is approximately 10 times larger than the Stockton Bar at 100 million cubic meters and detailed inspection of a boulder strandline offers a unique datum not normally preserved in the rest of the basin. This strandline is thought to have been created as the lake transgressed and transported finer sediments of the T-Q alluvial fan further along the barrier complex leaving a distinct boulder horizon overlain by Bonneville age sediments. Prior to this study, the eastern Deep Creek area was relatively undocumented, but is clearly important in understanding sedimentary processes and landforms along the western margins of Lake Bonneville.